text: There are a few real or perceived effects going on here:
  
  
  Elections to the European parliament have previously been described as second order elections, rather like local elections or the US midterms, with lower turnout, and more voting to "send a message" rather than to choose leaders. In particular, they haven't been very favourable to parties in government (note the collapse of the Liberal democrat vote, the junior member of the coalition).
  This effect was likely exacerbated by UKIP's near single issue mentality of withdrawal from the EU, and voters effectively treating the election as a pseudo referendum on EU participation.
  Different voting systems are used in the two elections, with general elections using single winner districts chosen by first past the post (plurality) voting, while the European parliamentary elections used multimember, closed list proportional system in England, Scotland and Wales (the D'Hondt method) and STV in Northern Ireland. This meant that a relatively small difference in vote share lead to a very significant difference in seats won.